Rutland and imperial suck. They get nice and hard and when you throw the heat to them they crack and fall off. How about high temp silicone?
I've used refractory cement with the same result. I suppose you could try Hi-temp silicone, but where will you be using it?
High temp silicone is only good to 600* F...so may only last for a while, depending on how hard you run. I like Meeco Red Devil furnace cement myself...but even that won't work the best sealing pipe seams. Why do you need to seal it up, pipe fit poorly?
I bought the Rural King brand, and although it suggests 24 hours before putting any heat to it (which I observed and followed), it is way better than Rutland. Very loose consistency tho, so a little challenging to apply.